The background of the invention will be set forth in two parts.
1. Field of the invention
The present invention pertains generally to the field of ceramics and more particularly to the field of down draft kilns.
2. Description of the Prior Art
The history of ceramics using kilns dates back probably earlier than 8000 B.C., the most advanced practitioners of this art being, until recent times, the potters of the orient. For example, there is evidence that kilns capable of producing temperatures above 1100.degree. C. were used in China by about 1000 B.C. These early kilns were usually made by digging a cave into the side of a hill, and were of the up-draft variety where the flames traveled upward, through the ware being fired, and out a flue. Later, Japanese kilns provided a cross-draft path which efficiently transmitted heat to the ware.
The kiln was further developed in China with the introduction of the first down draft kiln, where the opening to the chimney was at the floor level. In this way, the circulation of heat in the kiln was across and down through the fitting before exiting at the flue, which provided a rather efficient exchange of heat to the ware.
It was not until relatively recent times that down draft kilns were used in the Western world. The down draft kiln avoids much of the temperature variation problems found in other types of kilns and is generally considered to be the ultimate development in fuel burning kilns. A more complete treatise on the field of ceramics and the history, design and construction of kilns, may be found in suitable publications, such as for example, a book entitled "Kilns" by Daniel Rhodes, published by Chilton Book Company, Philadelphia 1968.
As natural and compressed gas became more readily available to both commercial and non-commercial users, relatively small efficient kilns have become popular in this country. However, a large majority of these smaller kilns have been of the upright variety because of their simplicity of construction and the fact that they do not require tall chimneys as are required for most conventional down draft versions. Gas has been found to be safe, easily burned, relatively cheap, and burners for this type of fuel are simple and inexpensive.
Basically, a kiln is a box or chamber of refractory materials which accumulates and retains heat directed into it. Heat may be transferred within the kiln to articles to be fired by any or combinations of three mechanisms, namely, conduction, convection, and radiation. In the conduction of heat through a solid, molecular activity of the solid is increased by the temperature elevation that is transferred from one molecule to the other. In solids, heat travels from regions of high temperatures to regions of lower temperatures, and the quantity of such heat transfer depends upon such variables as the thickness and area of the conducting surface, the temperature differential or gradient between the hotter and colder sides of the solid, and the nature of the material.
As to the transfer of heat through convection, a liquid or a gas moves because it becomes heated, and transfers its heat to something less hot. As applied to kilns, convection occurs when hot gases travel through a kiln, transferring some of their heat to all surfaces contacted by the moving gases.
The third way in which heat is transferred in a kiln from one body to another is by radiation. Heat creates electromagnetic waves which travel through space and induce heat in other objects upon which these waves are incident.
As noted previously, most present-day kilns of the small to medium variety are of the updraft type which have their flue opening located at the top of the kiln. It has been found that a kiln of this configuration cannot pull the heated atmosphere down through the center of the kiln, so that the heated gases mostly travel in only an upward direction and out the flue opening. This provides very limited circulation and thus this type of kiln is not efficient in transferring heat energy to the work to be fired.
On the other hand, most present-day down draft kilns utilize low velocity, horizontally oriented, gas burners that enter the kiln chamber through its side walls and require bag walls or heat deflectors to prevent scortching of the fired ware. The bag walls drastically cut the velocity of the circulating gases and significantly reduces efficiency. This low velocity configuration also requires a very high flue in order to create a large enough vacuum to pull the gases out of the chamber. Most such kilns require muffle tubes or saggers which give off radiant heat only, and no heat is transferred by convection. It should therefore be evident that a novel and yet simple and reliable kiln that produces a complete circulating atmosphere within the kiln chamber, and that does not require tall flues and bag walls, muffle tubes or saggers, would constitute a significant advancement of the art.